1. Field of the Invention
The present invention relates to a charged particle irradiation system and a method for controlling the charged particle irradiation system. The invention more particularly relates to a charged particle irradiation system that irradiates a patient with a charged particle beam such as protons or carbon ions and treats the patient, and a method for controlling the charged particle irradiation system.
2. Description of the Related Art
A treatment method is known for irradiating an affected part (cancer or the like) of a patient with a charged particle beam (ion beam) such as protons or carbon ions. A charged particle irradiation system used for such a treatment includes an ion beam generator, a beam transport line and an irradiation unit.
Examples of irradiation methods adopted by the irradiation unit include a passive irradiation method in which a beam is spread by means of a scatterer and is then extracted in conformity with the shape of an affected part; and a scanning irradiation method in which a fine beam is scanned across an affected part.
In a charged particle irradiation system using the scanning method, a charged particle beam is accelerated by an accelerator included in an ion beam generator and reaches an irradiation unit through a beam transport line. Scanning magnets provided in the irradiation unit deflect the charged particle beam for scanning. After that, the irradiation unit irradiates the affected part of the patient with the charged particle beam.
In this method, the extraction of the charged particle beam is stopped according to the accumulated dose of beams with which a subject is irradiated. With the extraction of the charged particle beam stopped, associated energy and the scanning magnets are controlled so that the position of a point (spot) to be irradiated with the charged particle beam is changed. After the position of the spot is completely changed, the irradiation unit restarts outputting the charged particle beam. Then, the irradiation unit irradiates the subject (affected part) while the position of the point to be irradiated is changed (refer to Japanese Patent No. 3681744, for example).
To prevent healthy cells from being exposed to the charged particle beam and perform an appropriate irradiation treatment without excessively or deficiently irradiating with the charged particle beam, a charged particle irradiation system described in Japanese Patent No. 3681744 has an irradiation unit that includes a beam position monitor and a dose monitor. The beam position monitor and the dose monitor are located on a downstream side of magnets and in front of a patient subject to be irradiated, and serve as irradiation dose detectors for measuring the dose of the charged particle beam with which the subject is irradiated.
In general, the beam position monitor is operated in such a manner that charges ionized by passing the beam are accumulated in a capacitor and a voltage induced in the capacitor after the spot irradiation is read out. The capacitance of the capacitor is determined so that the capacitor can store charges ionized when a spot is to be irradiated with the largest dose of the beam among expected doses. In the aforementioned method, the smaller the capacitance of the capacitor, the higher the resolution; and the larger the capacitance of the capacitor, the lower the resolution.
The charged particle irradiation system using the scanning method is configured as follows. The charged particle irradiation system divides a subject to be irradiated into some spots or points to be irradiated. The number of times of fractionated irradiation and an irradiation dose for each time of the irradiation are preset. The irradiation is performed on a single spot multiple times. Thus, a single irradiation dose (irradiation time) required for irradiation of each spot is reduced, and a variation in the irradiation dose is suppressed. The actual irradiation doses can be more reliably detected and evaluated (evaluation of the dose distribution and the like).
A control system for the charged particle irradiation system has an interlock. The interlock monitors failures that occur in the system and the constituent devices. In addition, the interlock monitors the state of the beam that is present in the accelerator. Even when a spot is being irradiated, if a failure occurs in the system or any of the constituent devices, the interlock is adapted to stop or interrupt the irradiation based on the type or level of the cause of the failure. In addition, the irradiation can manually be interrupted or stopped (or by pressing a button or the like).
Respiratory gating may be applied to the charged particle irradiation system in some cases. The respiratory gating is to interrupt and restart irradiation in synchronization with breathing of a patient. Specifically, the respiratory gating is such that beam irradiation starts and stops in response to a respiratory gating signal that is synchronized with breathing of the patient (when the signal is turned on, the beam irradiation is performed).
When a failure occurs or the respiratory gating signal is turned off during irradiation of a certain spot and its irradiation is interrupted or stopped, the irradiation dose (irradiation time) for the spot in question may be smaller (shorter) than a preset dose (preset time) depending on the timing of the occurrence of the failure. In consideration of characteristics of the aforementioned position monitor and effects of background noise and the like, it is difficult to accurately detect the position of the irradiated spot. As a result, it is difficult for an irradiation position detecting unit that includes the dose monitor to appropriately detect the actual dose of the beam with which the spot is irradiated. In addition, it is also difficult to appropriately evaluate the dose (evaluation of a dose distribution or the like). In this case, a single spot is irradiated multiple times with a smaller dose of the beam than the preset dose. Also, it is more difficult to appropriately detect and evaluate the actual irradiation dose of the beam with which the spot is irradiated. This will obstruct the system's efficient operation.
To avoid this problem, a charged particle therapy system described in JP-2008-237687-A does not immediately stop extraction of a charged particle beam (irradiation with the charged particle beam) when a relatively minor failure (in which continuous irradiation would be possible) occurs during the irradiation of a certain spot. More specifically, this charged particle therapy system is adapted to stop the extraction of the charged particle beam (irradiation with the charged particle beam) after the dose of the beam with which the spot is irradiated reaches a target dose.
Thus, even when a failure occurs during the irradiation of the certain spot, the irradiation is continuously performed on the spot. Therefore, it is unnecessary to irradiate a single spot multiple times separately (or it is unnecessary to interrupt and restart the irradiation), and it is possible to reliably irradiate the spot at one time. It is also possible to detect and evaluate (evaluation of the dose distribution and the like) the actual irradiation dose at a level similar to that at which normal spot irradiation is performed, while the actual irradiation dose (irradiation time) is not smaller (shorter) than a preset dose (preset irradiation time).